NAME

SYNOPSIS

DESCRIPTION

msocket() creates an endpoint for communication and returns a
descriptor in a multi-stack environment or defines the default stack.
The path parameter selects the stack used for the call. The path must
refer to a stack special file (S_IFSTACK). When path is NULL, the
default stack gets used. It is possible to specify a default network
stack for each domain (see SOCK_DEFAULT below).
The domain parameter specifies a communication domain; this selects the
protocol family which will be used for communication. These families
are defined in <sys/socket.h>. The currently understood formats
include:
Name Purpose Man page
PF_UNIX, PF_LOCAL Local communication unix(7)
PF_INET IPv4 Internet protocols ip(7)
PF_INET6 IPv6 Internet protocols ipv6(7)
PF_IPX IPX - Novell protocols
PF_NETLINK Kernel user interface device netlink(7)
PF_X25 ITU-T X.25 / ISO-8208 protocol x25(7)
PF_AX25 Amateur radio AX.25 protocol
PF_ATMPVC Access to raw ATM PVCs
PF_APPLETALK Appletalk ddp(7)
PF_PACKET Low level packet interface packet(7)
The socket has the indicated type, which specifies the communication
semantics or SOCK_DEFAULT to define the standard stack for the
specified domain(s). Currently defined types are:
SOCK_STREAM
Provides sequenced, reliable, two-way, connection-based byte
streams. An out-of-band data transmission mechanism may be
supported.
SOCK_DGRAM
Supports datagrams (connectionless, unreliable messages of a
fixed maximum length).
SOCK_SEQPACKET
Provides a sequenced, reliable, two-way connection-based data
transmission path for datagrams of fixed maximum length; a
consumer is required to read an entire packet with each input
system call.
SOCK_RAW
Provides raw network protocol access.
SOCK_RDM
Provides a reliable datagram layer that does not guarantee
ordering.
SOCK_PACKET
Obsolete and should not be used in new programs; see packet(7).
Some socket types may not be implemented by all protocol families; for
example, SOCK_SEQPACKET is not implemented for AF_INET.
When type is SOCK_DEFAULT msocket does not define any communication
endpoint, instead it defines the stack that will be used for the
following msocket calls with NULL path, or for the following obsolete
socket(2) calls. Default stacks get inherited through process creation
fork(2) and execution execve(2). When type is SOCK_DEFAULT and domain
is PF_UNSPEC the named stack becames the default stack for all the
protocols it supports.
The protocol specifies a particular protocol to be used with the
socket. Normally only a single protocol exists to support a particular
socket type within a given protocol family, in which case protocol can
be specified as 0. However, it is possible that many protocols may
exist, in which case a particular protocol must be specified in this
manner. The protocol number to use is specific to the “communication
domain” in which communication is to take place; see protocols(5). See
getprotoent(3) on how to map protocol name strings to protocol numbers.
Sockets of type SOCK_STREAM are full-duplex byte streams, similar to
pipes. They do not preserve record boundaries. A stream socket must
be in a connected state before any data may be sent or received on it.
A connection to another socket is created with a connect(2) call. Once
connected, data may be transferred using read(2) and write(2) calls or
some variant of the send(2) and recv(2) calls. When a session has been
completed a close(2) may be performed. Out-of-band data may also be
transmitted as described in send(2) and received as described in
recv(2).
The communications protocols which implement a SOCK_STREAM ensure that
data is not lost or duplicated. If a piece of data for which the peer
protocol has buffer space cannot be successfully transmitted within a
reasonable length of time, then the connection is considered to be
dead. When SO_KEEPALIVE is enabled on the socket the protocol checks
in a protocol-specific manner if the other end is still alive. A
SIGPIPE signal is raised if a process sends or receives on a broken
stream; this causes naive processes, which do not handle the signal, to
exit. SOCK_SEQPACKET sockets employ the same system calls as
SOCK_STREAM sockets. The only difference is that read(2) calls will
return only the amount of data requested, and any data remaining in the
arriving packet will be discarded. Also all message boundaries in
incoming datagrams are preserved.
SOCK_DGRAM and SOCK_RAW sockets allow sending of datagrams to
correspondents named in sendto(2) calls. Datagrams are generally
received with recvfrom(2), which returns the next datagram along with
the address of its sender.
SOCK_PACKET is an obsolete socket type to receive raw packets directly
from the device driver. Use packet(7) instead.
An fcntl(2) F_SETOWN operation can be used to specify a process or
process group to receive a SIGURG signal when the out-of-band data
arrives or SIGPIPE signal when a SOCK_STREAM connection breaks
unexpectedly. This operation may also be used to set the process or
process group that receives the I/O and asynchronous notification of
I/O events via SIGIO. Using F_SETOWN is equivalent to an ioctl(2) call
with the FIOSETOWN or SIOCSPGRP argument.
When the network signals an error condition to the protocol module
(e.g., using a ICMP message for IP) the pending error flag is set for
the socket. The next operation on this socket will return the error
code of the pending error. For some protocols it is possible to enable
a per-socket error queue to retrieve detailed information about the
error; see IP_RECVERR in ip(7).
The operation of sockets is controlled by socket level options. These
options are defined in <sys/socket.h>. The functions setsockopt(2) and
getsockopt(2) are used to set and get options, respectively.

RETURNVALUE

On success, a file descriptor for the new socket is returned except
when type is SOCK_DEFAULT. In this latter case 0 is returned on
success. On error, -1 is returned, and errno is set appropriately.

ERRORS

EACCES Permission to create a socket of the specified type and/or
protocol is denied.
EAFNOSUPPORT
The implementation does not support the specified address
family.
EINVAL Unknown protocol, or protocol family not available.
EMFILE Process file table overflow.
ENFILE The system limit on the total number of open files has been
reached.
ENOBUFS or ENOMEM
Insufficient memory is available. The socket cannot be created
until sufficient resources are freed.
EPROTONOSUPPORT
The protocol type or the specified protocol is not supported
within this domain.
Other errors may be generated by the underlying protocol modules.

CONFORMINGTO

This is a system call defined for View-OS. It extends socket(),
appeared in 4.2BSD and conforming to 4.4BSD, POSIX.1-2001. System
providing msocket() do provide also a socket() call for backward
compatibility. In fact: socket(domain,type,protocol) is equivalent to
msocket(NULL,domain,type,protocol)
In this way it is generally portable to/from non-BSD systems supporting
clones of the BSD socket layer (including System V variants).

NOTES

POSIX.1-2001 does not require the inclusion of <sys/types.h>, and this
header file is not required on Linux. However, some historical (BSD)
implementations required this header file, and portable applications
are probably wise to include it.
The manifest constants used under 4.x BSD for protocol families are
PF_UNIX, PF_INET, etc., while AF_UNIX etc. are used for address
families. However, already the BSD man page promises: "The protocol
family generally is the same as the address family", and subsequent
standards use AF_* everywhere.
The header file <sys/types.h> is only required for libc4 or earlier.
Some packages, like util-linux, claim portability to all Linux versions
and libraries. They certainly need this header file.

COLOPHON

This page has been modified from socket(2) page of release 2.79 of the
Linux. In fact msocket(2) is an extension of this call. man-pages
project. A description of the project, and information about reporting
bugs, can be found at http://www.kernel.org/doc/man-pages/.
View-OS is a project of the Computer Science Department, University of
Bologna. Project Leader: Renzo Davoli.
<http://www.sourceforge.net/projects/view-os>
Howto’s and further information can be found on the project wiki
<wiki.virtualsquare.org>.